This invention relates to an improvement in polymeric fatty acid polyamides useful in the flexographic ink industry, to provide products which will comply with environmental protection standards. The improvement lies in the use of acid termination and a sufficient amount of unsaturated monomeric fatty acids to provide a molecular weight product which will permit relatively high solids levels and reduced solvent emission while maintaining good ink varnish properties.
Flexographic inks are solvent based inks applied by rollers or pads to flexible sheets of plastic foil and paper. It is necessary that flexographic ink binders be found which will have certain properties. The practical aspects of the use of these ink resins and the inks derived therefrom require that the polyamide resin be soluble in alcohol solvents, and such solubility must be attained without sacrificing toughness, adhesion and gloss.
U.S. Pat. No. 3,253,940 was one solution to provide greatly improved solubility in alcoholic solvents, particularly ethanol. This was accomplished through the use of relatively short chain or lower aliphatic monobasic acids in the preparation of polymeric fat acids polyamides of diamines such as ethylene diamine. Illustrative of such lower monobasic acids were those having up to 5 carbon atoms such as acetic acid, propionic acid, butyric acid and the like. In U.S. Pat. No. 3,224,893 the hydroxy monocarboxylic acids were employed in such polymeric fat acid polyamides. Varnishes of the polyamides of these patents in alcohol solvents were on the order of 35% by weight non-volatile solids.
The foregoing provided resins which could be employed with the usual alcoholic solvents. However, as environmental solutions were sought, efforts were made to reduce emissions such as those from the volatile alcohol solvents. One means of reducing the emission was to provide water reducible polymeric fat acid polyamides as illustrated in U.S. Pat. No. 3,776,865. As disclosed therein, this was achieved by acid termination of the polymeric fat acid polyamides employing an acid component of the polymeric fat acid and another co-dicarboxylic acid and an amine component comprising isophorone diamine alone or in admixtures with conventional diamines such as the alkylene diamines, i.e. ethylene diamine. Acid termination was achieved by employing about 50-75 amine equivalents per 100 carboxyl equivalents. Varnishes of these resins in an alcohol solvent such as n-propanol on the order of about 40% nonvolatile solids are disclosed.
Another U.S. patent, U.S. Pat. No. Re. 28,533 dealing with polymeric fat acid polyamides employing lower aliphatic monobasic acids, such as acetic and propionic, with certain amine combinations disclosed a few solubilities in ethanol up to 60% percent though many were 50% or below.
As environmental standards have become more and more stringent, efforts have continued to provide resins which comply with such standards. High solids varnishes on the order of 55-60%, and preferably above 60%, which when formulated into pigmented inks will meet such solvent emission standards which are desired in order to reduce solvent emissions. The development of lower molecular weight resins will provide for higher solids varnishes. However, this must be accomplished without significant effect on other properties required for flexographic ink use. Such requirements generally include:
(a) good solubility in ink varnish-high solids p1 (b) very low viscosity at 60% solids PA1 (c) gel resistance PA1 (d) toughness--non-tacky PA1 (e) adhesion PA1 (f) gloss PA1 C.sub.18 monobasic acids 5-15% by weight; PA1 C.sub.36 dibasic acids 60-80% by weight; PA1 C.sub.54 (and higher) tribasic acids 10-35% by weight.